The long-range objectives of Research Project 1 are the identification, characterization, and validation of etiologically distinct subtypes or dimensions of learning disabilities. To accomplish these objectives, an extensive psychometric test battery will be administered to a sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair is reading disabled, to an independent sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair has ADHD, and to a comparison sample of 50 pairs of identical twins and 50 pairs of fraternal twins with a school history of no significant reading deficits or behavioral problems. In collaboration with investigators from Research Projects 2-6, resulting data will be used to assess the genetic and environmental etiologies of reading deficits and ADHD, as well as their covariation with measures of reading, language and perceptual processes, mathematics performance, executive functions, and brain morphometry. Reading performance data will also be used to test hypotheses of differential etiology as a function of age, gender, and cognitive ability, and to test the hypothesis that the etiology of deviant scores differs from that of individual differences within the normal range.